Field of the Art
The present invention relates to a high strength bolt and a method of manufacturing the same, and more particularly to a high strength bolt having a specific chemical composition and a manufacturing method therefor featured in heat treatment.
Related Art Statement
In recent years, remarkable tendency of lightening weight of automotive structural parts for the purpose of reducing fuel consumption naturally caused the necessity, also in the field of fastening bolts for fastening parts, to pursue high strength while demanding light weight.
When for example automotive parts or components become compact and of high strength, fastening bolts, such as connecting rod bolts and cylinder head bolts, for fastening those parts or components are necessarily required to be compact. It is quite natural that a small-sized bolt must be of high strength for maintaining its fastening capability.
Bolts of 12.9 class in the strength level, according to the ISO standard, have traditionally been utilized for such automotive-assembly use. Required strength standard conditions for such bolts of 12.9 class are: EQU tensile strength=120-140 kgf/mm.sup.2 ;
and EQU 0.2% proof stress.gtoreq.0.9.times.tensile strength.
Since the parts, which have been in harmony with bolts of just mentioned standard strength conditions, are now required to be more and more compact, bolts also have to catch up with the new demand for becoming smaller in size and greater in strength. This trend of the day demands appearance of higher strength bolts satisfying the conditions of ISO 14.9 class, that is to say: EQU tensile strength=140-160 kgf/mm.sup.2 ;
and EQU 0.2% proof stress.gtoreq.0.9.times.tensile strength.
Although there is stipulated, in JIS (Japanese Industrial Standard) as well as in ISO standard a high strenght bolt of 14.9 class in strength level, development of steel satisfying the necessary conditions for such a high strength bolt can not be said completed. That is to say, progress of the material for such a high strength bolt does not, as a matter of fact, satisfactorily follow the necessity of the present day.
Traditionally used bolt steel belongs to, as for its material quality, a Cr-Mo type steel such as JIS SCM440. It is well known that such a steel is remarkably deteriorated in the resistance to delayed fracture, when the tensile strength exceeds 120 kgf/mm.sup.2. This resistance to delayed fracture is in fact a key condition required for the bolts in automotive use, which must be improved by all means today. Steel which has been improved to a somewhat required level in the tensile strength, can not be practically used in places where the tensile strength of 140-160 kgf/mm.sup.2 level is actually applied, due to the deterioration of the resistance to delayed fracture.
An ideal steel, which is excellent in the resistance to delayed fracture and parallelly characterized in possessing features of high resistance to fatigue as well as high tensile strength, i.e., essential requirements to high strength bolts, has so far not been found.